Growth charts and Z-scores Prof DF Wittenberg
Children grow If they don t, there is a problem Standardized tables or charts are used to assess weight, length or height, skull circumference, and growth velocity
Factors affecting the individual s growth Abnormal intrauterine environment Genetic endowment Abnormal genes Timing of puberty Growth Nutrition Emotional health Endocrine disorder Chronic disease
Main influence on growth Infancy : (up to 2 years) Food/ nutrition Chronic disease Childhood Genes Growth Hormones Chronic disease Puberty Sex hormones
Measurement is the only way to recognize whether growth is normal or not Is this boy really 6 years old?
Length vs height
Measurement of stature Feet flat together against the wall Buttocks, Back against the wall Stand straight! Horizontal mark opposite top of head Measure against the wall
Measurement of growth Length or height measurement Example of a little boy s data: eg Age Ht 2.5 yrs 92 cm 3.0 yrs 95 cm 3.5 yrs 97 cm 4.0 yrs 99 cm 5.0 yrs 101 cm
Previous slide s data plotted on the graph Growth charting identifies the divergence from the normal Growth chart is only way to recognize whether growth is normal or not
The most important feature of growth charts A measure to compare and monitor the physical status of an individual child with the childhood population on an ongoing basis. Different types of growth charts Road-to-Health Chart : mainly a weight for age chart Longitudinal growth charts Percentile graphs Z score graphs BMI charts Weight for length/height charts Velocity charts Prenatal or combined pre- and postnatal growth charts Special populations : Down syndrome, Turner syndrome
There is a normal range
www.who.int/growth
Percentile or Z-score graphs Of 100 healthy children at a given age: 3 per cent have height measurements less than the 3rd percentile 97 of 100 children have measurements below the 97th percentile 50 per cent above or under the 50th percentile 50th percentile measurements correspond to the mean and median Z-scores or SD scores are used to describe mathematically how far a measurement is from the median (average). The mean (median) is the same in both types of graph
Range of normal 50% 50% 97% 50 th percentile corresponds to a Z score of 0 3 rd percentile corresponds to Z score of 1.72 97 th percentile corresponds to a Z score of + 1.72
How to calculate Z scores (Standard Deviation Score) Example: Actual length 96.1 cm Median 90.4 cm Standard deviation 3.3 Z score = 5.7/3.3 = +1.73
Comparison of growth charts Percentile graph Z score graph Percentile graphs have a narrower range, because the 97 th centile corresponds approximately to a Z score +2 A Z-score of +3 or -3 is more likely to be definitely abnormal
NCHS or WHO charts Growth charts are established on large populations of normal children living under near-optimal conditions and therefore representing the range of normal growth for children at different ages. NCHS : USA population, cross-sectional New growth standards have been developed by the World Health Organization (WHO) based on the growth of normal breast fed infants in various regions of the world. International growth reference standard
Growth of breastfed babies Pooled data from affluent countries Growth patterns of BF babies similar in different populations Slower weight gain from age 3-12 months Deviation of growth occurs when babies are already on solids, ie not deficiency Less effect on length than weight At 2 years average weight close to reference Dewey KG, et al. Pediatrics 1995; 96 (3): 495-503
Comparison of breastfed growth with CDC growth curves From : WHO 2006
Dewey et al Pediatrics 1992; 89: 1035-1041
Final height of breastfed babies Slow growth in second 6 months of life despite additional solids Other factors than breastfeeding per se apparently responsible for associations Final height is no different from formula-fed babies Girls 165.3 vs 164.9 cm Boys 175.3 vs 175.8 cm Zadik et al J Ped Gastroenterol Nutr 2003
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Example of data shift by wrong measurement: 8 years old. Weight 31kg Height 1,23 m: BMI = 20.5 If height is actually 1.26 m: BMI = 19.5 Remember influence of measurement on derived numbers
After measurement, what next? Measurement does not improve growth Interpret the graph Action must follow: Any child with Z score < -3 Any child crossing the lines Weight/height discrepancy Consider wasting
Interpret the growth parameters
Velocity normal Velocity slow Patient s measured length Height age Chronological age
Abnormal potential Growth velocity normal Abnormal Outcome Growth velocity abnormal Chronic disease Malnutrition Endocrine disease Normal potential